require or disallow trailing commas (comma-dangle)

The --fix option on the command line can automatically fix some of the problems reported by this rule.

Trailing commas in object literals are valid according to the ECMAScript 5 (and ECMAScript 3!) spec. However, IE8 (when not in IE8 document mode) and below will throw an error when it encounters trailing commas in JavaScript.

varfoo={bar:"baz",qux:"quux",};

Trailing commas simplify adding and removing items to objects and arrays, since only the lines you are modifying must be touched.
Another argument in favor of trailing commas is that it improves the clarity of diffs when an item is added or removed from an object or array:

Less clear:

var foo = {
- bar: "baz",
- qux: "quux"
+ bar: "baz"
};

More clear:

var foo = {
bar: "baz",
- qux: "quux",
};

Rule Details

This rule enforces consistent use of trailing commas in object and array literals.

"always-multiline" requires trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }

"only-multiline" allows (but does not require) trailing commas when the last element or property is in a different line than the closing ] or } and disallows trailing commas when the last element or property is on the same line as the closing ] or }

Trailing commas in function declarations and function calls are valid syntax since ECMAScript 2017; however, the string option does not check these situations for backwards compatibility.

You can also use an object option to configure this rule for each type of syntax.
Each of the following options can be set to "never", "always", "always-multiline", "only-multiline", or "ignore".
The default for each option is "never" unless otherwise specified.